The present invention relates to refrigeration at very low temperatures, less than 1.degree. K.
It relates more particularly to a novel industrial product which is a small-size helium 3 hermetic refrigeration stage, of a reduced cost and static in character, i.e., comprising no moving parts (except possibly one or two heat switches) and being adapted to be readily incorporated in most conventional helium 4 metal cryostats, thus extending their field of use to about 0.3.degree. K.
The stage of the invention is suitable for example for maintaining at a cryogenic temperature, less than 1.degree. K., the sensitive element of an infrared region detector, particularly for experiments in the far infrared.
Our U.S. Pat. No. 4,136,526 discloses a portable helium 3 cryostat, comprising, disposed inside a portable helium 4 cryostat of a known type, a unit having an evaporation chamber containing in operation liquid helium 3, a reservoir at a level above the chamber, a first duct connecting the reservoir and the evaporation chamber, an adsorption chamber containing an adsorbent which absorbs helium 3 only below a critical temperature higher than that for vapporizing helium 4, and a second duct which connects the adsorption chamber to the first duct, a valve being disposed in the second duct, or at the inlet or outlet thereof, so as to be able to seal the adsorption chamber from the sub-unit including the reservoir, the evaporation chamber and the first duct, said unit, hermetically sealed, containing a gaseous mass of helium 3 which is highly compressed at ambient temperature.
This cryostat which performs very well and is easy to use exhibits however for some applications the disadvantage of a relatively high cost and requires the use of a helium 4 cooling apparatus with a removable bottom for receiving the helium 3 cryostat having the improvements in accordance with our patent.
Helium 3 cryostats are moreover known for use with detectors in the far infrared region making use of pumping by means of active charcoal. A cryostat of such type is described for example in an article by Junya Yamamoto published in the "Japanese Journal of Applied Physics," vol. 14, No. 11 (November 1975), pages 1807 to 1810, entitled "A He.sup.3 Cryostat Using a Charcoal Adsorption Pump for a Far-Infrared Detector."
This cryostat comprises, in a vertical metal tube, an He.sup.3 bath in its lower part, a condenser in its middle part and a mass of active charcoal in its upper part with a heating coil surrounding said mass. The unit is placed into an He.sup.4 cooler.
For desorbing the He.sup.3 gas out of said mass, the tube is evacuated, which requires a pump, and the mass of active charcoal is heated; on the other hand, adsorption is carried out by stopping the heating and the He.sup.3 condenses on the walls (cooled by the He.sup.4) of the middle part of the tube and falls to the bottom thereof while replenishing the He.sup.3 bath in the lower part of the tube. It has been found that gravity plays a role, which prevents the cryostat according to this Japanese article from being operated in a position other than a substantially vertical position with the active charcoal in the upper part. Furthermore, this cryostat is immersed in a bath of liquid He.sup.4 (which is pumped); therefore, it must be introduced into an He.sup.4 cooler which must be constructed to allow such introduction.
A cryostat similar to the one of YAMAMOTO is described in an article by WALTON, TIMUSK and SIEVERS entitled "A compact He.sup.3 cryostat using activated charcoal" in The Review of Scientific Instruments, volume 42 (1971) pages 1265-66.
Another cryostat substantially of the same type and having the same requirements of orientation and immersion in an He.sup.4 cooler is described in an article by D.B. Tanner published in the Physical Review B, vol. 8, No. 11 (1 December 1973), pages 5045 and following and entitled "Fluctuation Contribution to the Far-Infrared Transmission of Lead Films" (see in particular pages 5046 to 5048 insofar as the cryostat is concerned).
The present invention aims at providing a helium 3 cryostat not having this requirement. In fact, it does not necessarily have to be used in a certain orientation with respect to gravity and it need not be introduced into a helium 4 cooler.